The present invention relates to an improvement of an adaptive auto equalizer which automatically compensates distortion and noise contained in a signal propagated through a transmission line, thereby equalizing the signal to an ideal signal and outputting the ideal signal.
Generally, adaptive equalization refers to a process of shaping the waveform of an input signal into that of an ideal output signal, based on a predetermined adaptive equalization algorithm. According to the process, a signal distorted by the effects of various noises during propagation through a transmission line is restored to an original ideal signal.
The structure of a conventional adaptive auto equalizer is shown in FIG. 25. This drawing shows a transversal filter block 1 and an adaptive control block 2. The transversal filter block 1 includes delay units 11, 12, 13, and 1N, multiplication units 21, 22, 23, and 2N, and addition units 31, 32, and 3M. The adaptive control block 2 includes a determination unit 41, an error detection unit 51, and a coefficient amendment unit 61.
The transversal filter block 1, which is a well-known FIR (Finite Impulse Response) filter, convolutes a plurality of data delayed for a respective predetermined time period to an input signal, and outputs the result. To be more specific, the transversal filter block 1 enters a signal distorted during propagation through a transmission line, and each of the delay units 11, 12, 13, and 1N delays the signal for a predetermined time period. Then, each of the multiplication units 21, 22, 23, and 2N multiplies each delayed data with a corresponding weight coefficient, and the addition units 31, 32, and 3M serially add each multiplication result together and outputs the sum outside.
The adaptive control block 2 serially modifies the weight coefficients used in the multiplication units 21 to 2N, and the weight coefficients change themselves with time. The serial modification of the weight coefficients are necessary for adaptive equalization because in such a system as a hard disk or a compact disk for recording a plurality of signals in high density, an occurrence of inter-code interference from an adjacent bit causes non-liner distortion of the waveform of a signal or a decrease in the amplitude of the signal. In the adaptive control block 2, the determination unit 41 compares a signal outputted from the transversal filter block 1 with a predetermined threshold value, so as to determine and output the expected value of the signal. The error detection unit 51 finds the error between the signal outputted from the transversal filter block 1 and the expected value outputted from the determination unit 41. The coefficient amendment unit 61 calculates the weight coefficients of the multiplication units 21 to 2N of the transversal filter block 1, based on the error value found in the error detection unit 51, and serially modifies the weight coefficients. By repeating this operation, the output signal of the transversal filter block 1 is made closer to the expected value.
However, in the adaptive auto equalizer shown in FIG. 25, the transversal filter block 1 and the adaptive control block 2 are both ideal, so that the update of each weight coefficient is performed without any delay and the output signal of the transversal filter block 1 is converged upon the expected value at an early stage. In actuality, however, the operation of each of the multiplication units 21 to 2N, the addition units 31 to 3M, the determination unit 41, the error detection unit 51, and the coefficient amendment unit 61 includes a delay amount. As a result, it is necessary to update a weight coefficient in a time period corresponding to the sum of these delay amounts. In other words, it is necessary to use the same value of a weight coefficient in the time period corresponding to the total delay amounts. Consequently, there is a problem that it takes long to converge a weight coefficient upon the expected value, which indicates poor convergence.
On the other hand, it is possible to calculate and update a weight coefficient in the coefficient amendment unit 61 for each of the signals entered sequentially. However, appropriate amendment of a weight coefficient relies on obtaining the output signal of the transversal filter block 1 by using the previously updated weight coefficient, so that if the update of a weight coefficient is performed for each of the consecutive signals, the output signal of the transversal filter block 1 is not converged upon the expected value, which results in oscillation.